1. Field of the Invention
The present invention relates to electronic devices, and more specifically to a method for securely transferring data in a programmable circuit.
2. Description of Related Art
In some applications, it is desirable to protect data elements contained in a memory of a programmable circuit from the possibility of being inspected while being transferred to another memory. For example, the secret data elements can be personal data elements that identify the owner of the programmable circuit, program instructions, or keys for data encryption algorithms. The secret data elements are usually stored in read-only memories of the programmable circuit during fabrication. Conventional techniques are used to protect the contents of such read-only memories from a visual inspection. For example, the data elements may be scattered in the memory. However, when the data elements are used, they travel in non-encrypted form on a data bus that can be easily snooped.
In one conventional snooping technique, the current flowing through the bus is measured. This current represents the data that flows through the bus. The snooper makes K measurements of current during K transits of the same data elements, and takes the average of these K measurements to eliminate the noise from the measurement and obtain the exact value of the data element. In general, it is necessary to make about 1000 measurements (K=1000) to remove the noise and obtain the exact value of the data traveling through the bus. This snooping technique is known as “Simple Power Analysis”.
Furthermore, in order to reduce the cost of manufacturing products, the secret data elements are often partly the same for a given family of programmable circuits. Thus, if a snooper manages to read the secret data elements stored in one product, they can be used for an entire family of products.